As a fuel injection control device for a diesel engine with an EGR device, a fuel injection control device is proposed, for example, in the patent publication JP-A-9-195825, in which the concentration of oxygen in an intake gas flown into the cylinder is detected with a sensor, the amount of oxygen therein is computed from the result of the detection, and then the maximum amount of fuel injection necessary to suppress the amount of generated smoke to the tolerable limit is determined based on the computed amount of oxygen. Other prior art documents regarding the present invention include JP-A-9-126060, JP-A-9-4519, and JP-A-10-37786.
The amount of generated smoke correlates with the combustion speed in the cylinder. The combustion speed varies according not only to the amount of oxygen in the intake gas but also to the composition of the intake gas. That is, even if the same amount of oxygen is contained in the intake gas, the combustion speed slows down and smoke is generated more easily, for example, when the partial pressure of a molecule such as CO2 and H2O having large specific heat increases as the EGR ratio increases. The conventional fuel injection control device detects the concentration of oxygen and uses the detected concentration of oxygen only for computing the amount of oxygen. However, the conventional fuel injection control device does not take the variation of the concentration of oxygen into consideration when the smoke tolerable limit value is determined. Accordingly, the control of combustion speed regarding smoke suppression may not be performed accurately enough.